In practice, the function of a linear motion electric motor, for example the motor disclosed in U.S. Pat. No. 4,602,174, is to drive a mechanical element such as the piston of a compressor with controllably variable amplitude oscillatory motion. To accomplish this, the amplitude of the voltage applied to the motor winding must be controllably variable.
If the source of electrical power for the linear motor is constant voltage AC such as 120 VRMS, 60 hz., controllably variable voltage to energize the motor can be generated inexpensively with a triac, using, for example, the circuit disclosed in U.S. Pat. No. 5,592,073.
If the source of electrical power for the linear motor is DC, e.g. a battery or solar panel, voltage from the source must be converted to variable voltage AC to energize the motor. DC to variable voltage AC conversion has been accomplished in prior art by using pulse width modulation (PWM) technique, one example of which is disclosed in U.S. Pat. No. 5,156,005.
A PWM linear motor driver applies to the motor winding a train of rectangular voltage pulses of alternating polarity, at a repetition frequency much higher than the oscillation frequency of the motor, and with a duty cycle controllably modulated at the motor oscillation frequency so as to cause the average value of a pulse to vary with its time of occurrence. The motor responds to average value but has negligible response at the pulse repetition frequency. Therefore, by modulating pulse duty cycle at the required frequency of motor oscillation and with variable amplitude, controllably variable amplitude AC voltage can be effectively applied to the motor.
A disadvantage of a PWM motor drive is that it can be a source of serious electromagnetic interference (EMI) unless it is provided with costly shielding and filtering. EMI is generated by PWM drivers because of high speed switching needed in PWM to minimize switching losses. Typical PWM switching time is 150 nanoseconds, which can generate EMI in the radio and television bands. Since devices equipped with DC powered linear motors may be used near radios or TV sets, very effective EMI suppression is essential to such devices and adds significant cost.
The cost of a PWM motor driver is increased by its requirement for electronic switches capable of high speed switching, which are needed if switching losses are to be acceptably low. Field effect transistors (FETs) are usually used, but are relatively expensive compared to bipolar transistors, which are generally inapplicable to PWM because their relatively slow switching would cause excessive switching loss.
Accordingly, the object of the present invention is to generate, from a DC source, controllably variable voltage AC to drive a linear motor, and to do so without generating significant EMI and with the option of using either FETs or inexpensive bipolar transistors as switching elements in the invention; the choice of switching element depending on considerations of cost, efficiency, and available DC voltage.